


Smutty Side Fics for Undertaken: A Tale of Two Sisters

by Dark_Crystal_Demon



Category: Undertale (Video Game)
Genre: ...to a skeleton, Fluff to Smut, Loss of Virginity, Read if you wish your virginity had been taken by a skeleton., Side Story, not cannon
Language: English
Status: In-Progress
Published: 2016-10-07
Updated: 2016-10-07
Packaged: 2018-08-20 01:07:42
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 3,160
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/8230978
Author URL: https://archiveofourown.org/users/Dark_Crystal_Demon/pseuds/Dark_Crystal_Demon
Summary: I love Undertaken: A Tale of Two Sisters. This story fills my heart with hope and darkness. It's not every day I get into OC pairings, but Mimi and Aly fill me with determination.....And naughty thoughts in regards to their relationships with skeletons. While thelovelyskelesisters keep their fic classy, I can keep my explicit thoughts to myself no longer. With the author's permission, I'm allowed to post an excerpt from the fic at the beginning for a little context, but I highly encourage everyone to read this fic and fall in love with these sisters like I have. If you're also a huge fan of Undertaken and harbor dirty thoughts of hidden scenes, share them in the comments.





	

**Author's Note:**

  * For [thelovelyskelesisters](https://archiveofourown.org/users/thelovelyskelesisters/gifts).
  * Inspired by [Undertaken: A Tale of Two Sisters](https://archiveofourown.org/works/6779137) by [thelovelyskelesisters](https://archiveofourown.org/users/thelovelyskelesisters/pseuds/thelovelyskelesisters). 



Excerpt from [Undertaken: A Tale of Two Sisters](http://archiveofourown.org/works/6779137?view_full_work=true) Chapter 12 by thelovelyskelesisters:

“Sans, look at me,” Mimi called out desperately. Her voice was cracking too, breaking under the pressure of finding any reason to make him stay. She didn’t want to be alone right now. She needed someone.

She needed him.

“Kid, please…”

Maybe it was just as easy as asking.

“Sans…” her voice was still shaking, making it difficult to get the words out. “Will you…”

Finally, he turned back around and patiently waited for her to finish.

Anxiety quaked through her bones and tied her stomach in knots. But she had to. She had to ask. “Will you just... stay with me tonight?”

Sans’ eyes shrunk to pinpricks. He opened his mouth to speak, but no words came out.

Mimi could feel the tears again. She already knew the answer. God, she was so stupid! Maybe the embarrassment would kill her if the heartache didn’t.

Sans stared at her for the longest moment before taking his jacket off and tossing it onto the floor.

“Is that… is that really what you want, Mimi?”

It was already too late to stop the tears at this point. But underneath her swollen red eyes and uncontrollable weeping, she nodded.

“...P-Please… don’t go…” she gasped in between sobs.

Just as quickly as he had been prepared to leave, Sans was now back at her side again. He tenderly held a hand to her cheek and wiped away what tears he could. There were too many for him to keep up, but he tried. He always tried, and that was enough.

“...Not going anywhere, kid.”

 

~*~*~*D*~*~*~*C*~*~*~*D*~*~*~

 

His words echoed against Mimi's heart as his fingers brushed her tears away. She leaned into him. Head buried in his shirt as he stroked her hair and let her cry.

She didn't know how long they sat that way.

His shirt was soaked, and her legs were falling asleep. She couldn't cry anymore. And no amount of crying was going to save her sister. Aly had disappeared and Mimi was powerless to stop it. She had no magic, and no idea what to do next. She shook her head against Sans' clavicle and sighed, turning herself in his arms and leaning her back against his ribs. Every thought was overwhelming.

His arms were wrapped around her waist in comfort but...

     Every thought...

She fit perfectly between his legs, against his pelvic bone.

     ...was overwhelming.

"Do you feel better, kid?" Sans asked with a hug around her waist; completely unaware of the shift in her thoughts. Mimi blushed, nodding enthusiastically and trying not to make it obvious that her stomach was doing flips under the pressure of his ulna and radial bones.

His arms continued to hug her. He wasn't letting go. His skull leaned against her shoulder blade. His voice muffled as he talked into her back, "Your heart... it's racing..."

Mimi tried to calm herself, she covered her arms over his, to hug him back. She dropped her head, hair covering her face; even though he couldn't see her from behind. She was embarrassed by her own emotions. She should only be worrying about her sister. Not-

"Mimi?" his mouth was by her ear. His voice had dropped. It made her heart jump into her throat. She couldn't speak with the pounding in her chest as became more aware of his body around hers. 

She shivered and gripped the bones around her waist. His hands found hers, arms still crossed around her body. He was so warm. The magic holding him together permeated into her skin where she touched him. It made her body feel like it was glowing. 

"Sans... could you ever... date a human?" Mimi whispered, barely audible, but she knew he heard her by the way his body stiffened and his hands tightened around hers in surprise. 

She closed her eyes and braced herself for his answer. His fingers were the first to move, twining themselves around hers. Flesh and bone. Rough bone, scarred a dozen times from cut marks he's never explained. 

"It's too dangerous here," they'd already had this discussion but...

"It's dangerous everywhere."

"You need to get Aly out of here."

"No," Mimi let go of his hands. She turned herself around, kneeing in front of him, balancing herself with a hand on each of his legs. She locked eyes with his and took a deep breath of courage before she said, "Not when we've finally found a home."

"Mimi..." Sans cupped her face in his hands and she leaned forward. 

It felt so natural to kiss a skeleton. She had imagined it a thousand times, but she never thought it would come true.

He was gentle and sweet. The lights in his eyes were glowing blue embers. She could feel his soul beat against her skin through his bones. Her heart joined in the rhythm and she smiled for the first time in days.

"You're so beautiful," Sans whispered, pinching her cheeks. "...and soft."

Mimi blushed under the tips of his phalanges. He marveled at her ability to look so cute and sexy all at the same time. He brushed a finger across her lips. To his surprise, she opened her mouth and closed it around him, a blush still on her cheeks as she sucked on the tip of his bone.

"Oh Mimi..." It took every once of will he had not to throw her down on the bed right then.

Mimi smiled at Sans reaction and continued to lick his finger, tracing the micro-fractures with her tongue. She imagined healing his old scars with her saliva, healing him as she repaired her own confidence.

He removed his finger and traced a wet trail down her jaw line and along the muscles in her neck.

With everything that had happened since she entered the underground...

...everything had gotten so out of her control...

... she seized his lips with hers and imprinted them with his skull. Flesh and bone, it felt like she could melt around him. She slid her hands over is mandible, circling his temporal bones to his auditory meatus. He shook under her touch and let his hand dare to venture below her neck...

"Hhhhgn," Mimi gasped against his mouth as he gently traced the outline of her breast through the fabric of her dress. He circled around her nipple and she became hard at his touch. Her breath caught against his mouth as he pinched and twisted her between the tips of his phalanges. "Sans..."

The lust in her voice was a welcome new tone that Sans had only ever heard in his dreams, sitting on the couch, imagining her... and now here she was. Her eyes begging him for more as she kneeled between his legs. She was blushing, smiling, gasping his name.

Oh Mimi...

He was going to savor her.

...Every part she would let him.

He was going to take a _long time_...

Sans tangled his fingers in her hair as he took her whole breast in his hand. Mimi purred as he massaged her head and fondled her at the same time. His mouth claimed hers again and she darted her tongue between his teeth and up along the inside of his maxilla. The taste of bone and magic sent an electrical current along her tongue and down her throat. 

She had never imagined he would taste so good. And then she felt something slippery against her lips. She broke the kiss and saw blue light between Sans' teeth. 

"Is-is that a tongue?"

Sans licked his lips playfully with a smug smirk. He bent his head down to her other breast and moistened it with his mouth. Mimi gasped as the fabric of her dress clung to her, soaked in his magical saliva while he continued to fondle her.

Mimi's fingers curled around his occipital bone at the back of his skull when he started to suck on her nipple. Sans sucked harder as her fingers unknowingly tightened around his sensitive region. He was seeing stars as she clung to his occipital. It made his tongue move faster, but he bit down on her breast when she squeezed the bone on the back of his skull in excitement. 

"AH!" 

Sans gave her a sheepish grin, looking up at her from between her breasts. "Sorry, sensitive bones."

"No, um," Mimi's face became really red. She knew he saw her as pure but would he judge her if... "I-I liked it."

"You can't be real," Sans desire rose out of his control and against the confines of his pants. It just grazed the tops of her knees; Mimi's eyes widen as she understood what was pressing against her skin. Sans bent his head back down and bit her again. Her back arched and pushed her knees forward just enough to rub his tip through the fabric of his pants. He bit her again and again. Trying different places. Finding new sensitive areas around her breasts, avoiding the bruises on her neck...

Mimi wanted more. Her dress felt too confining. She grabbed the hem and practically tore it over her head. Her boobs bouncing at their new freedom, her skin prickled as the cool air of the room surrounded her. Then she realized she had just made herself practically naked in front of Sans. 

The hungry look in his eyes aroused feelings that dampened the only piece of clothing Mimi was still wearing. No one had ever looked at her like that. It was exhilarating as Sans admired his own handiwork, her skin decorated with tiny red marks. His marks.  

"You're beautiful." He cupped both of her breasts, skin soft beneath his bones. She moaned, and Sans knew that every noise of hers was his. Every arch of her back. Every quiver of her lip. Every smile. Every glance. 

"Oh Sans..." She was his.

He slowly replaced his hands with his mouth as he encircled his arms around her back and slowly eased her onto the bed. One arm under her head, the other exploring the softness of her skin as they kissed, hips pressed hard against each other as their tongues danced. 

Mimi became bolder by the minute and started her own exploration under his shirt. It was hard to concentrate. Sans was so skilled that her hands shook most of the time. She did figure out how sensitive his ribs were, but when she squeezed his xiphoid, he ground his hips into hers. She moaned as the bulge in his pants rubbed against the damp fabric of her panties and along her clit. 

Wow. 

She gripped his ribs with pleasure as he pushed himself against her again. 

"Oh my..." Sans grabs her hips, fingers hooked onto the band of her panties as he slowly rubs himself up and down between her legs. His eyes watch her reactions. Making sure she's comfortable with his advances. She just kisses him harder, not wanting him to stop.

They rock, pleasure unfolding with each grind as their hands continue to explore each other more desperately. She rips off his shirt and relishes the feel of his ribs against the skin of her breasts. 

Mimi instinctively raises one leg around his hips to give him better access and herself leverage to keep the rhythm going herself. 

"Mi...Mi..." Sans' voice shakes as she presses against him harder. His breathing is getting shorter as she speeds up with the pleasure building between her legs. "W-we...ah...um... d-don't-tah ... ah... have... to... AH..."

Mimi nods, understanding as she feels herself coming against him. She lets out a loud yell and is grateful Sans doesn't have any neighbors. Sans smiles and enjoys the ride as she spasms against him.

Mimi blushed, realizing what she just did but Sans doesn't seem to mind. He cuddles her against him and kisses her lightly on the forehead. No pressure. This would be enough. Her happiness. He tried to scoot his hips back so they wouldn't press against her and make her feel like he had to finish. 

Mimi looked in his eyes, filled with a love he had yet to announce, but she knew. She always knew. She felt her soul glow in recognition of its mate. 

She kissed him and she felt as if her heart jumped out of her chest and into his, merging. She felt whole and an urge within her wanted to feel that wholeness in a different way. 

"Sans," Mimi grabbed the belt loops in his pants and pulled him against her. "Be my first."

"Mimi," Sans' voice was mixed with desire and hesitation. "Are you sure?"

"I love you, Sans," Mimi confessed, feeling relief that she had finally allowed herself to say it. It was the truth. No matter what his answer.

His human. He hadn't let himself feel this way in a long time. He told himself he wouldn't. 

"You stole my heart the first day I met you." Sans knew it was foolish... "I've been trying to get it back ever since." He knew this path could only lead to pain... "You're the brightest soul I've ever met." But he couldn't let her slip away... "I love you so much, Mimi."

Can a heart break from happiness? In dark times, was this allowed? Would they pay for their sin of joy? He kissed her and left those thoughts behind as she pulled him on top of her. 

He kissed her harder. Her hips rose to meet his. Her hands shook a little as they tried to unzip his pants. He stopped her, "Not yet."

Mimi bit her lip and gripped the pillow beneath her head as Sans trailed tender kisses down her body. A bite followed a kiss every few inches that made her gasp, but as he fingered the edge of her panties, she lost the ability to breathe. Anticipation held her frozen as he slowly removed her last piece of clothing and kissed every spot between her legs. 

He paused between her legs and looked up past her breasts to her face. "You're sure?"

"I trust you," Mimi said, knowing this was exactly what she wanted. A bright moment of love when everything else was spiraling out of control. She needed this. She needed him. There was no one she trusted more in the world right now...

"Oh!" His tongue came out and licked her lips between her legs. She felt like ice cream melting against his tongue.

"Mmmm," he hummed against her skin and she spread her legs wider in his hands. His tongue applied more pressure and slid apart her lips. She moaned as he probed deeper, drinking her. His tongue seemed impossibly long, but thin. Gently caressing her tight inner walls.

He used a hand to circle gently around her clit as his tongue began to enlarge, stretching her ever so slightly.

"W-whoa," Mimi felt light-headed as he swirled his tongue slowly, gently opening her wider as his hand kept a steady rhythm. Waves of pleasure surrounded her and inside her. "Sans."

She started moaning his name and once she started, she couldn't stop. 

"Oh Sans."

His human...

"Oh Sans."

His delicious human...

"Oh...Oh..."

Sans slowly removed his tongue and unzipped his pants at last. He looked at her with that Oh face as he continued to circle her clit. Her hair was splayed all over the pillow, breasts heaving, stomach tightening in need, hips jerking into his hands, legs spread...

He grabbed his tip and traced it along her entrance. She squirmed trying to take him inside her, but Sans kept outside, teasing her more. He loved how she begged. "Sans, please."

Sans placed just the tip inside her walls as he positioned himself on top of her, kissing her as she kissed him below, squeezing her muscles around him. It took all his control to keep himself from thrusting into her. But he had a girthy member and she was so tight... he had to take this slow.

He started to push past the tip. She nearly screamed at how big he was. He pulled out just as slowly, covered in her wetness and his own pre-cum. 

"Are you okay?"

"You feel amazing!" Mimi moaned and Sans re-entered her, going just a little deeper this time. "Oh, Sans!"

He pulled out again. It was getting difficult to control himself. She was so tight and he wanted her so bad. When she yelled his name...He re-entered, deeper, she stretched around him, squeezed him as he encountered her spot. Her legs wrapped around his femur and tried to push him further, "Oh, deeper!"

Sans took a breath. He didn't want to hurt her, but she kept pushing and finally he slid all the way inside. 

"SANS!" Mimi came just from the sensation of being so completely full. He stayed still as she rode herself around him. Slick and tight. He couldn't get over how tight she was or how sexy her primal side was as she came again against his cock. When he pulled out, her virginal blood was there.

What a gift. She was a gift. She was... incredible.

He couldn't stand it. He started his own rhythm as she hummed in the plateau of her orgasm. He stimulated her clit again and taught her about her third orgasm and her fourth. Each one made it harder for him to hold on, but this was her first time. It had to be special. 

He understood her rhythm now and timed his own orgasm with her next. As she squeezed around him, he let himself lose control, pounding against her as she moaned unable to form words anymore. He came, bursting inside her with a force that left him weak, collapsing on top of her with kisses. 

Sans rolled off of her and then pulled her fleshy body against his bones. They laid there, dazed, and out of breath. 

"Mimi, are you...okay?" Sans realized he had gotten a little rough right at the end.

"Mmmhmm," Mimi still couldn't form words. She felt... amazing. She could feel his magic dissipate inside her with a tingling sensation as it left. It still felt like he was inside her. It was like he left an imprint of himself. So this was what it felt like. She was so happy she could cry. She nuzzled his clavicle and kissed him. 

Her smile... he would do anything for her smile. 

She closed her eyes, exhaustion finally taking her to sleep. He watched her peaceful breaths and covered them both with his blanket. 

"Mimi, I will protect you," he whispered in her ear. Then he looked at the glow stars on his ceiling and thought about the monster who spoke with his hands. He covered his blind eye with his own hand and thought of Aly. "I'll protect you from him. And, if I have to, from your own sister."

 

**Author's Note:**

> Again, thank you thelovelyskelesisters for giving me permission to do smutty things to your main characters. I've done my best to portray them as honestly as I could.
> 
> Also, please note that's I have no idea how this fic is going to end. I'm not an author of Undertaken. So I took some creative liberties with certain descriptions and, obviously, Mimi/Sans' relationship.
> 
> Check Out My Tumblr (NSFW): [Dark_Crystal_Demon](https://www.tumblr.com/blog/darkcrystaldemon)
> 
> And if you really like my style, subscribe to me at [Dark_Crystal_Demon](http://archiveofourown.org/users/Dark_Crystal_Demon), to get updated when I make new fics or chapters!


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